Gear Ratio

[hemogoblin]

Hi Emery,
I've been following this thread and it occurred to me that you had a new
Jasper engine. Since they spec a special cam in the GMC engines, would the
Torque & HP curve be the same? Perhaps a check with Jasper about their Cam
Grinder might be in order.
Mark '77 Palm Beach LINY

 

[brent covey]

Hi Again all;

Several owners were kind enough to write with weight data for thier GMC's

The consensus thus far is 26' models seem to be approaching 12,000 lbs with
great regularity. Adding a toad would add another couple thousand pounds.

I do not have conclusive data for the 23' models however. Being lighter but
of identical frontal area, they should play it conservative, likely the same
axles suggested here for the 26' would be good enough.

After doing the math for different possible combinations, heres what I'd
suggest.

3.07- unacceptible.

3.21- acceptible for very light coaches used outside mountainous areas. Was
standard ratio in 1966-67 Toronado, and 1967 Eldorado.

3.42- preferred ratio for general purpose use, in coaches not used to tow.

3.55- ideal for heavier coaches, mountainous areas, high speed driving.

3.67- even better! Minimum for best performance with a toad.

To flesh out these numbers a bit, your GMC engine if installed in an 1973
Olds Delta 88, which weighs 4600 lbs approximately, would tow 2000 lbs with
a 3.08, 4500 lbs. with a 3.23 and 7000 lbs with a 3.42.

L78-15 tires are fitted to the big Olds with trailer package, and these are
very slightly less rolling diameter than the GMC wheels.

In effect, Olds feels for 11,000 lbs gross combined weight to be powered
adequately, a 3.42 is needed.

Chevrolet rated thier trailering special Pickup truck of that era with the
454 engine and 3.73 axle at 14,700 lbs GCVW. A 4.10 ratio was optional on
1/2 tons, and a 4.56 on 3/4 and one ton trucks and Suburbans.

So, to sum, I'd say at least a 3.42 is worthwhile. Bearing in mind that you
were expected to drive around day to day between towing events with the 3.42
in this hypothetical Oldsmobile, and not experience any troubles, it isn't
too far to go. My own experiences with other vehicles suggests that 3.73 -
3.90 would be a point of diminishing returns for fuel economy.

One thing that strikes me is how bad the fuel consumption reported by most
people is. I assume most must be driving very fast indeed! For the amount of
energy being consumed to propel a GMC 60 MPH, one should expect to see in
the vicinity of 12 MPG at minimum in reasonably flat and low wind
conditions. I tow a car hauling trailer of 7500-9000 lbs (depends what I've
got on it) with a 454 Crew Cab truck (~6200lbs) and 3.73 ratio, and see low
13's normally at 60-65. Solo the truck will top 16 mpg at 60. It runs 2600
rpm @ 60.

I feel the higher numerically axles should if anything, improve fuel
economy, and drastically so in winds, rolling terrain or mountains.

As far as camshafts et al in Olds 455 engines, thats worth watching. Most
"RV" grinds on camshafts are not helpful. Use the stock replacement cams,
they're still availible, and the best overall comprimise in my view.
Changing the camshaft will change torque peaks.

Bear in mind, the torque peak essentially means the engine speed where it is
most efficiently filling the cylinders, and hence has the highest pressure
before combustion. This translates in real terms into the point the engine
will be most hyper sensitive to spark knock! The factory distributor is
calibrated to flatter this point by not being too advanced in this
particular region. Changing cams throws GM's very accurate and painstaking
calibration in this department out the window. You can of coyurse recurve a
distributor to flatter the new cam profile, but thats a lot of work, and few
people are capable of doing so. If the cam and distributor are badly
matched, you'll end up having to retard initial advance to keep it from
knocking, and there goes all your economy, and much of the power.

One last thing to remember as a general principle- most hot rodders and
'performance' minded people are striving to improve HP figures at elevated
engine speeds. This is not helpful to us. High horsepower is a very
secondary consideration to being able to summon up lots of energy at low
speeds to heave the GMC along and overcome the weight, and maintain a high
amount of output continiously to buck the substantial aerodynamic forces in
such a large body.

Horsepower is torque over time- hence, an engine with little torque turning
fast can make quite extraordinary HP figures. There are many small engines
in the two litre class now that make peak HP close to what the 455 does. I'd
remind you its the area under the HP curve that represents the total power
availible, and not the peak figure. Hot rodders try to move the torque peaks
UPwards in the RPM range, and hence trade much of the peak torque away for
greater gains in HP. In other words, they lose substantial flexibility.

Hope this is helpful

Brent

 

[brent covey]

Hi Arch!

Good points, and I'll try to address each for you.

>I am running a stock Olds distributor. The other day I took out the 23
degree
>advance and replaced it with the 10 degree vac advance. The only
differences
>in these two units is the length of the slot in the stationary arm. I have
>installed a shift kit in my tranny which gives me a shift point of 22 mph
in
>first gear and 42 in second gear.

So far so good- the shift points are not especially important from an
ignition timing point of view- essentially the engine should be able to
adjust its timing to suit whatever you throw at it.

>When I drop in what ever gear ratio I end
>up with this is going to put me well up in the RPMs. My thought is that I
>might
>be able to let the engine get a little more advance out of the vac unit.
All
>that would have to be done is lengthen the slot a little.

You may, here's a little explaination for you of what is happening-

The centrifugal advance is set up essentially to provide maximum advance the
engine will tolerate at FULL THROTTLE, i.e. maximum cylinder pressures. The
pressures peak at more or less the torque peak of the engine, and fall off
either side of that. Looking at a graph of torque output for an engine will
give you a very accurate idea of the relative pressures in the cylinders at
wide open throttle. Generally speaking, the centrifugal advances are
extremely well set up at the factory.

Its an idea to remove the vacuum advance hose and plug it, and set the
engine for maximum tolerable static advance or nearly so at WOT without
pinging. The factory setting is usually quite close to this. Note also,
engines develop greater sensitivity to spark knock as the temperature rises.
A feuls ability to avoid pre ignition is largely dependent upon three
factors. Temperature, Pressure, and Octane rating. Around the 225 degree
water temperature mark, the sensitivity is substantially increased in most
iron block gasoline engines. Pinging MUST be kept to an absolute minimum,
and must never occur at heavy throttle. It breaks pistons and can damage
many other parts.

Now, the vacuum advance is designed to take advantage of the substatially
lower peak cylinder pressures at part throttle operation. When a cylinder is
not operating at maximum pressure, the engine can tolerate considerably
greater spark advance and reap the benefits of cooler operation, greater
economy and greater power. The problem is always finding a Vacuum Advance
unit that does not overdo it. This usually calls for a bit of
experimentation. Bear in mind also, altitude plays a big part. At higher
altitudes, the peak pressures are somewhat lower, and you may discover a
engine that works perfectly in high altitude knocks and runs poorly at sea
level.

Anyhow, basically, the order to calibrate the engine is, is tune for the
worst.

Use 87 octane fuel, at the lowest altitude you normally encounter, set the
timing to spec, or as advanced and as close to it as possible with the VA
disconnected for ZERO knocking on a hot engine at FULL THROTTLE.

Next, advance the timing from this point four-six degrees. Hook up the VA
unit again. If the engine is not on the hairy fringe of knocking as you
drive at light throttle, you need a more agressive VA. If it does knock at
light throttle, but it diminishes at very heavy throttle, your VA is too
agressive. The ideal VA would keep the engine on the verge of knocking all
the time with the initial advance slightly to far ahead. The idea is to have
a responsive VA that is just a little less agressive than the engine can
stand. Naturally, reset the initial advance back where we wanted it when
thru.

If you can find one that meets these criteria, you'll be very happy with it.
I buy all the ones off similar distributors I encounter and experiement to
match whatever vehicle I'm driving. One advantage I enjoy is living at sea
level, if it doesn't knock here, it is never going to elsewhere;-) Even two
identical model and part numbers Vacuum Advance units can differ
substantially due to production variances.

> Add to that the use
>of
>premium gas and I might even be able to go back to the 23 degree advance
>and get much better gas mileage. All of this is just theory right now. All
I
>know
>for sure right now is that the only pinging I was getting with the 23
degree
>advance was when I crowed it in third----but I am real lite right now since
>there is nothing in the coach. Sure would like to hear your thoughts.

I think one real benefit of the numerically high axle ratio is the lessened
strain and load on the engine will permit cruising at substantially higher
vacuum readings, which will bring the VA back on line. Driving with 14-18"
HG Vacuum at a few hundred RPM more is desirable if it means the distributor
can advance and the carb enrichment circuit remains inoperational more
often.

I wouldn't use premium or tune for it. The 455 is not touch and go by any
means and can make the maximum power on 87 if its tuned well. You're very
near that point already. Also, if something does go wrong, and you for
example got sold some real no-test bad gas, being tuned for 87 will leave
you some options, such as throwing some premium in to boost the bad stuff.

>I agree with you on the higher gears. But that seems to be something that
>is not all that good here. When I look at some of these motorhomes
>running a 454 geared in the low to mid 4s I wonder what we are doing.
>There was one article about combining a Caspro chain set with some of
>these gear sets to get you to a 3.9 I think it was. That looked interesting

You are correct in noticing Brand "X" motorhomes on Chevy 454 Chassis use
4.10-4.88 ratios frequently.

There are in my opinion a few reasons for this, primary reason is most use
quite a bit larger tire (larger tires have fewer revolutions per mile) than
the GMC, and hence need a higher ratio to get the same results. Also, most
SOB's are less aerodynamic, have greater weights and more rolling resistance
due to inherent inefficiencies like dual rear wheels and tall chassis
configurations. In short, they need all the help they can get.

There are some differences in the Olds 455 and Chevy 454 engines also, as
used in the two Chassis types. Olds has left thier engine in passenger car
tune essentially, and hasn't really recalibrated it beyond stripping out a
few emissions control devices. The Chevy 454 in the Motorhome Chassis is a
heavy duty engine, purpose built to be slow and steady in its performance,
and have an extra flat torque curve. 454 engines came in several versions,
the 454 versioon representing closest overall equivalent of the 455
Oldsmobile in a GMC is Chevrolet's LS5 option, essentially a run of the mill
1971-72 Chevy Caprice engine.

The contrast between the Motorhome 454 and 455 GMC is essentially that the
455 is more like car to drive. It has greater horsepower, and likes to rev
a little more than the 454.

Basically, as long as you arent geared so short you are continously
operating over about 3500 rpm at the abslute highest cruise speed you expect
to drive, you wont hurt anything. Gearing so that the bulk of your cruising
mileage occurs at or slightly below the torque peak (~2400-2800 rpm) will
usually yeild the best overal fuel consumption.

I personally will want at least a 3.64 for any GMC I get. The Caspro
PowerDrive chain looks like a helluva nce arrangement, lower ratio with no
tradeoffs elsewhere( except $$ I guess!) . If I could get a 3.73 or 3.89
etc, I'd be sorely tempted to do so.

Thanks for the kind remarks Arch! Hope this is food for thought

Brent Covey
Vancouver

 

[brent covey]

Arch;-)

> Brent are you looking at this?

I sure am;-)

I think the 'best' single measure is the PowerDrive, if I had the choice of
only one modification.

That would get you into the efficency zone on its own, and is likely to have
the greatest change for the dollars spent and least possibility of problems.
The PowerDrive is substantially less apt to tax the skills of a mechanic
than a ring and pinion during installation.

The next choice would be to use a PowerDrive *and* the 3.42 gears. A 3.90
would be about the lowest ratio I'd imagine would be useful to most people.
If you can't tow or move it with 3.90's, it wont be the gearing, it'll be
the front tires cant pull it. At stall, you'd have very nearly 9000 ft/lbs
of torque applied to the front wheels with a stock 455, 3.90 final drive and
switch pitch. You only have ~6000 ft/lbs of tractive power! Contemplate that
figure;-)

3.90 would be about 2750 rpm + slip at 60 MPH, 2950 @ 65 MPH This would be
real good for a heavy unit, or someone who has a toad, in my view. At 55
mph, economy should be amazing with this arrangement. The difference in
performance would be similar to what having a 575 cubic inch engine with a
stock axle ratio might be like. Or another 75 HP output. You'd be running a
little too fast at speeds over 65, but the engine would sure appreciate the
break, and good mileage would be as easy as slowing down a little.

>Question Emery at what RPM would I be running the 455 at with the
>3.90 going 65 MPH?

>> Power Drive gives a 3.5 ratio with the standard 3.07 final drive. With
a
>> 3.21 (that Caspro also sells) it would be 3.21/3.07*3.5= 3.66. With a
>3.42
>> it would be 3.42/3.07*3.5= 3.90. With a 3.55 it would be 3.55/3.07*3.5=
>> 4.05. For a 3.67 it would be 3.67/3.07*3.5= 4.18.
>>
>>Emery

 

[donald w. miller]

There has been much discussion of increasing gear ratios but a rather
elegant alternative has yet to be explored and considered.

How about reducing weight instead ?

Many GMCMH are operated well over manufacturers gross weight rating.

Weight reduces performance and increases maintenance problems.

Increasing gear ratio from 3.07 to 3.42 only increases performance
marginally.

With all else equal, it seems a 10,770 lb. unit with 3.07 gear would
accelerate and climb grades similar to a 12,000 lb. unit with 3.42 gear.

With all else equal, the lighter unit with 3.07 will be clearly superior in
some other areas such as:

On level terrain and downhill terrain
Less chance of blowouts
Better braking
Better fuel mileage
Less engine, drive train, wheel bearing, brake, shock and tire wear

For the past 20 years large trucks and automobiles have lost weight and
their top gear ratios have decreased rather than increased. Both of these
factors have contributed to improving fuel mileage and reliability.

If net members applied our talents to finding ways of getting some fat out
of our fleet my guess is the results would be astonishing.

New units would undoubtedly be lighter if GM were doing this project now
instead of 1970.

If we visualized what at 2000 model GMCMH would be like, it would provide us
some good ideas.

Just like GM designers did in the 70's, our thinking must go beyond what is
offered by competitive products.

Today, many newer SOB's seem to be the same tired old overweight designs
with a new set of fancy stripes on the exterior and maybe some new gee whiz
gimmicks.

It's late, I'm tired, and I hope this makes some sense. Someone correct me
if my thinking is flawed.

regards,

Don Miller
75 project
Shenandoah Valley of Virginia

 

[vic marks]

Don Miller has raised the issue that I have been thinking about for the
past couple of years. I have a 75 transmode, which I am in the process of
gutting. I have done the suspension work (JA brakes, Caspro Shocks, Addco
Sway Bars, Variable ratio steering box - still need to reinforce the
A-arms and replace all of the rubber bushings and do the wheel bearings)
which made a tremendous difference in the driving experience (especially on
mountain roads). I intend (shortly) to raise the body by two inches (which
gives me about 30% extra capacity for custom made tanks) and install a
fresh water tank and gray-water tank (in addition to the black water tank
that I have at present). Muffler will be at the rear. By raising the body
and moving the fresh water tank lower (as well as adding a gray-water tank)
I will actually lower the center of gravity (and improve the handling). In
addition, I will take out the 400 pound Onan (see Jim Anstett's early
articles in GMCMM), remove the AC from the roof and put a new (lighter) one
in the rear compartment (as done by Jim Anstett) or mount it in the middle
of the coach. The interior will be replaced with lightweight cabinets etc.
Not only will the weight be reduced significantly (I estimate by 2000
pounds) but it will be shifted forward (batteries to the front etc.) giving
me better traction in bad (snowy) weather. The reason for the reinforced
A-arms (as Buskirk does for his stretches) is to handle the additional
weight up front. Having talked with a number of people about the bearings,
I would agree with Brent (if I have understood him correctly) that the
problem with the bearings is not the bearings but the race that they sit
within (assuming I have the terminology correct). A different grease may
solve the problem or perhaps some higher tech materials or tighter
tolerances around the bearings.

My gear ratio thinking is that I would like to go to a higher (meaning
lower numerically) gear ratio. I have a 3.07 gear set. I'd like to put
larger tires (I'm running 225-75 Michelins on the front) so that I can get
an even higher gear ratio. I would then be able to run comfortably in
second gear up to 60 miles an hour. I continue to be puzzled by various
people's attitude to second gear. It is as if it was a sin to shift down a
gear to maintain a steady RPM (a very North American attitude as opposed to
the European mentality about this issue). That should handle any uphill
problems (assuming that the engine is set up to maximize torque in the
2500-3000 rpm range). My assumption is that I could run in 2nd gear at
55mph all day long if I so choose without causing extra strain on the
transmission. I have on hand a switch pitch torque converter (67 Toronado)
to put together the 6-speed transmission but there are serious heat
problems to solve on that one. With the high ratio gears, I don't expect
acceleration problems as I'm not planning on drag racing it (although I am
thinking about the Bonneville land speed record being held by a class C RV
- - anybody want to talk about getting a rig set up to reclaim this record? -
it may be that high ratio gears and light weight might be just the ticket
for this). By minimizing weight and running the high ratio gears I think
that I can improve my gas mileage by 50% (and if Don is correct, cause less
wear and tear on the drivetrain). Assuming that I do improve the gas
mileage and I drive 100,000 miles over the next ten years (I'm in my late
forties, so I'm hoping to put on 250,000 miles before the GMC & I call it a
day), and assuming an average price of $1.50 US per gallon (I have to keep
the currencies straight as I'm Canadian), and assuming 8 miles per gallon
without these alterations, then I can potentially save $6000US per 100,000
miles. The real question is whether the size and shape and weight (even
reduced) ultimately prevents the mpg from going up to 12 on an averaged
basis.

Please critique this ladies and gentlemen.

Vic Marks
Vancouver
75 transmode

There has been much discussion of increasing gear ratios but a rather
elegant alternative has yet to be explored and considered. How about
reducing weight instead ? Many GMCMH are operated well over manufacturers
gross weight rating. Weight reduces performance and increases maintenance
problems.
Increasing gear ratio from 3.07 to 3.42 only increases performance
marginally. With all else equal, it seems a 10,770 lb. unit with 3.07 gear
would
accelerate and climb grades similar to a 12,000 lb. unit with 3.42 gear.

With all else equal, the lighter unit with 3.07 will be clearly superior in
some other areas such as:

On level terrain and downhill terrain
Less chance of blowouts
Better braking
Better fuel mileage
Less engine, drive train, wheel bearing, brake, shock and tire wear

For the past 20 years large trucks and automobiles have lost weight and
their top gear ratios have decreased rather than increased. Both of these
factors have contributed to improving fuel mileage and reliability.

 

[brent covey]

Vic wrote;

>Please critique this ladies and gentlemen.

Rather than quote the long part back, just a couple observations.

Regarding using second gear more, I definitely would say most drivers dont
make the most of it! I certaiinly think it can be used at ANY road speed
under 70 MPH for periods of climbing grades and to keep the transmission
cool, and road speeds up. It main economy advantage is climbing hills. If
you are in third gear and at 7" vacuum and shift to second and are at 9",
you're saving substantial amounts of fuel, as this will lift you out of the
enrichment zone. As soon as a carb power enrichment opens, you begin to use
about two-three times the fuel you did just out of it.

Minimizing the time spent near 7"Hg vacuum is my main reason for suggesting
high numerical ratios. Keeping the engine running near the torque peak pays
quite large dividends in overall efficiency. Now, a Fuel Injected engine as
example, can bend the rules a little, and operate at extremely low vacuum
and speeds under a heavy load and still return good economy, as well as
reaping the benefits of reduced throtling losses (less losses just take
create manifold vacuum) and lessened frictional losses from lower engine
speeds. Unfortunately, the best example of this thinking is the 1981
Cadillac 368 V8 with the V8-6-4 system. I have one, and yes, it works, but
at what a cost!

Saving weights as mentioned is a good way to save fuel and make the most of
the power availible. However, reality is, most people are looking for ways
to increase weights by not only adding lots of goodies to the coach, but
pulling toads behind and stuff like that. The same overall guidelines will
allow a person to find a good match for axle ratio whether they are
attempting to move 8,000 lbs or 16,000. Once its rolling, the question
becomes more aerodynamic in any event, and weights are not a key
consideration.

I'd be a little leery of operating a THM 425 transaxle under continous
service in second gear. I make very liberal use of second, and have had no
problems, but I've also had my nose in those transmissions on several
occasions and been very suprised what minimal provisions are made for
carrying end play on planet carrier gears. I think wear would be extremely
rapid if you drove very far. Suppose theres no substitute for just trying it
out however;-) Might be pleasantly suprised. In any event, running the
output thru yet another set of gears is an additional inefficiency, and
probably should be avoided in the long run.

My calculations for drag plotted on engine brake specific fuel consumption
information suggest somewhere between 14-16 MPG Imperial represents best
theoretical real world economy a vehicle with the GMC's aerodynamics and
weight will likely be able to obtain at highways speeds around 60 mph, using
an engine like GM has supplied us. So, theres definitely room somewhere for
some imprvement on the order of 25% in fuel economy potentially possible. My
view is better matched final drive gearing will be the solution, at least
for me.

I'm very interested in seeing your coach Vic, particularily the suspension
additions. Its been a steep learning curve here on the GMC, but I am
completely impressed with the engineering which is first class for the most
part.

Regarding wheel bearings, I'm going to pass this along to the engineers at
my company for thier evaluation. Basically, those bearings are far from
approaching load capacity, yet seem to fail a lot. This usually indicates
faulty lubrication, or bad installation, but perhaps something else is
happeneing, like flex in the knuckles or something. One possbility is that a
better bearing that addresses such a potential flaw in the GMC could be
designed. Anyhow, I'll see what they think and let you know.

One last thing- why are you concerned about heat from the switch pitch? The
periods the converter spends in 'hi stall' mode are very short, and
generally on the balance, the extra acceleration afforded by the reduced
ratio more than is compensation for the slight extra heat- namely, you get
up to cruise speeds so much quicker that the actual transmission oil temps
never rise far.

I've got switch pitch units in two cars at present, and am extremely pleased
with them, if you'd like perhaps I could have you over to try one out under
a towing situation so you could gain a feel for what they do, if you hadn't
yet had an opportunity.

Anyhow, excellent post!

Brent Covey
Vancouver

 

[travis martin]

I continue to be puzzled by various
> people's attitude to second gear. It is as if it was a sin to shift down a
> gear to maintain a steady RPM (a very North American attitude as opposed
to
> the European mentality about this issue).

Hello, Vic,

An interesting concept. As to the second gear question, I really don't
think it will matter much if you use second occasionally for hills, etc, as
Zak suggested, but I suspect you will have real problems trying to use it
for extended cruising.

In the TH400 and TH425 transmission, first and second gear are reduced with
a planetary gearset. The planets and the pilots of the input shafts run in
bushings. Third, on the other hand, is nothing more than the direct clutch
pack locking up and connecting the input shaft to the output shaft. I have
an idea that extended running in second will wear out these bushings in a
relatively short time, and I would also think that extended use of a
planetary reduction set would tend to foam the oil.

I do not have any first hand experience with this; I am just "thinking out
loud." If anyone has actually driven for long periods successfully for a
long period, I'd love to hear about it.

Travis

 

[brent covey]

Emery writes/quotes etc;-)

>3.90 would be about 2750 rpm + slip at 60 MPH, 2950 @ 65 MPH T >>
>
>Chuck - we've both posted rpm numbers for various gear ratios but they have
>had some differences. I used 2300 rpm at 60mph with a 3.07 as a starting
>point. J.R.Wright had reported 2275 to 2350 as the range. It appears that
you
>may be using 2200 rpm. Any feel for actual amount?

My figures deliberately do not reflect torque converter slippage, as thats a
variable, and would introduce errors that might make direct comparisons more
difficult.

Torque converters will slip a small amount usually under most circumstances
once they've passed 'stall' speed (1600-2300 dependent on the unit you've
got, generally) and they are generally in the range of 100-250 rpm
difference between driven and drive sides of the converter. The actual
amount of slip is largely dependent upon how much HP you are pushing thru
it. At heavy tirottle they slip more of course.

This means you could add about 200 rpm or so to the figures I quote for
engine RPM @ road speed for a slightly more accurate picture. So, both sets
of numbers are basically correct, just one suggests the final speed. This
doe not really affect the criteria for axle ratio selection however, as at
part throttle, the speed at which the engine torque peaks occur is elevated
slightly also.

>Also you have mentioned slippage of the torque converter. Do you feel that
>you would get more slippage with a lower ratio (numerically higher) than
you
>would get with a 3.07? I suspect that there would be very little, if any
>difference.

Well, I'd say you would get less- a GMC with say a 3.90 only needs about 75%
the torque of a standard ratio coach to maintain the same road speeds, and
this would mean sightly less slippage at the converter. How much precisely
is speculation, but it would be a bit.

Brent Covey
Vancouver